The invention relates to a linear drive without a piston rod having a guide tube which has a longitudinal slot leading through its wall and in which a piston is mounted so as to be displaceable in the longitudinal direction.
Linear drives without piston rods generally have a guide tube, in which a piston is mounted in a slidably displaceable and sealed-off manner. To guide the force generated as a result of the action of a pressure medium on the piston out of the guide tube, the latter has a lateral slot, through which a web connected to the piston passes. The slot or longitudinal slot extends at least over the entire path to be covered by the piston and, in general, over the entire length of the guide tube. It is therefore necessary, in order to form pressure-medium chambers, to seal off the longitudinal slot in the regions in which the piston is not exactly standing in each case. A sealing band is used for this purpose, the sealing band bearing on the longitudinal slot from inside and being seated with a portion in the latter. In the region of the piston, the sealing band is lifted off inward from the longitudinal slot, in order to make way for the web which surrounds the sealing band. In order to make it possible for such linear drives actuated by a pressure medium to work reliably, it is necessary for the sealing band to be held near the longitudinal slot, so that it does not sag or fall inward.
DE 3,429,783 A1 discloses a linear-drive cylinder, in which the sealing band sealing off the longitudinal slot has two ribs or lips which are arranged parallel to and at a distance from one another and which pass through the longitudinal slot and are each provided on the end face with a so-called bead. Both the sealing band and the ribs consist of flexible material. The ribs are pretensioned resiliently outward, that is to say away from one another in the spreading direction, and engage with their beads behind the longitudinal slot, in order to fix the sealing band in the longitudinal slot.
In this linear-drive cylinder, the material used for the sealing band determines the spring properties of the lips and therefore how securely the sealing band is held on the guide tube. Moreover, the ribs and beads project from the longitudinal slot, and this must be taken into account when the linear-drive cylinder is installed in machines and plants.
Furthermore, the above-mentioned document laid open for public inspection discloses a linear-drive cylinder, in which the longitudinal slot has flanks which are provided with a longitudinal groove, into which the sealing band engages by means of corresponding longitudinal beads.
Here too, the design of the sealing band and the choice of material for the latter must take into account the desired flexibility required for guiding the sealing band into and out of the longitudinal slot.
Moreover, EP 0,260,344 A3 discloses a pressure-medium cylinder with a longitudinally slotted guide tube, in which a sealing band bearing from inside on the guide tube near the longitudinal slot cooperates by catching with a covering band bearing from outside on the longitudinal slot. For this purpose, the sealing band has two outwardly directed longitudinal webs which extend parallel to and at a distance from one another and are curved slightly away from one another and which between them limit an interspace. The sealing band engages into this interspace by means of a rib provided in one piece on the sealing band and pointing radially inward, a clamping or catching connection being obtained as a result of the inclination of the faces which are in contact with one another.
In this pressure-medium cylinder, the covering band is additionally required as well as the sealing band. Both bands have to be lifted off from the longitudinal slot near the web and the web must be guided around the two bands.
Moreover, it is known from the above-mentioned patent application to hold the sealing band and covering band together by means of a magnetic strip, corresponding ferromagnetic steel bands being provided both in the sealing band and in the covering band.
In this embodiment of a pressure-medium cylinder, in addition to the sealing band and the covering band, the magnetic strip also has to be lifted out from the longitudinal slot, in order to allow passage to the web connected to the piston.